next.js/test/integration/404-page-app/pages/_app.js

import App from 'next/app'

function MyApp({ Component, pageProps, extraProp }) {
  return <Component {...pageProps} extraProp={extraProp} />
}

// Only uncomment this method if you have blocking data requirements for
// every single page in your application. This disables the ability to
// perform automatic static optimization, causing every page in your app to
// be server-side rendered.
MyApp.getInitialProps = async (appContext) => {
  // calls page's `getInitialProps` and fills `appProps.pageProps`
  const appProps = await App.getInitialProps(appContext)

  return { ...appProps, extraProp: 'Hi There' }
}

export default MyApp